Organic field effect transistors (OFETs) have features such as low cost and good flexibility because organic transistors can be manufactured by a coating process such as printing. Accordingly, organic field effect transistors are expected to find new applications different from applications of conventional Si transistors, for example, flexible displays and RFID (radio frequency identification) in which information is transmitted and received by near field communication, for example, using electromagnetic fields and electric waves. Organic semiconductors used in organic field effect transistors, however, have low carrier mobility and thus suffer from problems such as small current modulation and low response speed.
In order to solve these problems, vertical organic transistors have been proposed. The vertical organic transistors have a laminated structure comprising a collector electrode and an emitter electrode as upper and lower electrodes, an organic semiconductor held between the collector electrode and the emitter electrode, and an intermediate electrode provided within the organic semiconductor. The laminated structure has advantages, for example, shortening of channel length carried by the organic semiconductors, for example, to not more than 1 μm, high-speed response and enhanced power by virtue of effective utilization of the whole electrode surface, and lowered susceptibility to the influence of the interface. Accordingly, high-speed response at a low voltage can be realized even when an organic semiconductor having low carrier mobility is used.
In the vertical organic transistor, upon the application of a fixed voltage across the emitter electrode and the collector electrode that are provided respectively as opposed upper and lower electrodes while holding the organic semiconductor therebetween, current hardly flows between the emitter electrode and the collector electrode when voltage is not applied across the emitter electrode and a base electrode provided within the organic semiconductor, whereas, when voltage is applied across the emitter electrode and the base electrode, the amount of current which flows across the emitter electrode and the collector electrode is significantly increased to allow the current to be modulated.
As described, for example, in patent documents 1 to 4 “Japanese Patent Application Laid-Open No. 327615/2004, Japanese Patent Application Laid-Open No. 27566/2007, Japanese Patent Application Laid-Open No, 243871/2005, and Japanese Patent Application Laid-Open No. 324203/2003 [Claim 1]” and non-patent document 1 (Kudo Kazuhiro, “Recent progress and future trend of organic transistors”, OYO BUTURI, Vol. 72, No. 9, p. 1151-1156 (2003)), the conventional vertical organic transistor comprises an organic semiconductor and striped (meshed) intermediate electrodes (which act as the base electrode) inserted into the organic semiconductor. In these embodiments, the organic semiconductor is in contact with both the emitter electrode and the collector electrode and also exists in “opening” between the striped intermediate electrodes. Accordingly, current which flows through the opening defined by the organic semiconductor is modulated by applying voltage across the emitter electrode and the intermediate electrode.
For example, in the patent documents described above, the pitch (stripe pitch) of the striped intermediate electrodes provided at given intervals in the lateral direction is large and several tens of micrometers, and the width of the organic semiconductor present in the opening between the intermediate electrodes is also large and several tens of micrometers. Therefore, the area influenced by the voltage applied between the emitter electrode and the intermediate electrode is disadvantageously limited to a part around the edge of the striped intermediate electrodes. In order to effectively modulate the current which flows through the whole area of the opening defined by the organic semiconductor from the viewpoint of overcoming the problem, it is reported that reducing the spacing between the striped intermediate electrodes (for example, to about 100 nm) is effective (non-patent document 2: Satoru Toguchi, et al.; “Extended abstracts (The 53rd Spring Meeting, 2006); The Japan Society of Applied Physics and Related Societies”, 23a-ZG-3, p. 1412 (2006)).
In non-patent document 2, however, the fine pattern of the intermediate electrode is formed using microfabrication means such as electron beam lithography. Accordingly, the cost incurred in the manufacture of the vertical organic transistor is increased, and, thus, the technique disclosed in non-patent document 2 is not practical. The problem leads to the loss of the advantage of the vertical organic transistor that the channel length can be reduced by a simple manufacturing method.